1. Field of the Invention
The present invention relates to a heat pump apparatus, and more particularly to a heat pump apparatus which can enhance the defrosting efficiency of an evaporator and suppress variation of the optimum refrigerant filing amount when the temperature of water being supplied is varied in a heat pump type hot water supply apparatus designed so as to supply hot water heated by a gas cooler.
2. Description of the Related Art
There is known a heat pump type hot water supply apparatus that generally has a refrigerating cycle including a compressor, a gas cooler, a pressure reducing device and an evaporator and is designed to supply hot water heated by the gas cooler.
For example, when CO2 refrigerant is used as a refrigerant in a refrigerating cycle in this type of apparatus, it is expected that a high coefficient of performance (COP) can be achieved in a heating process having a large water-temperature rise-up range because there is established a transcritical cycle in which the high pressure side is set to a supercritical state in the refrigerating cycle.
In this type of heat pump type hot water supply apparatus, when the temperature of water supplied to the gas cooler is low, for example, about 5° C., CO2 refrigerant discharged from the compressor is heat-exchanged with low-temperature water of about 5° C. in the gas cooler and cooled to a low temperature until it has the same level density as liquid (hereinafter referred to as a liquid state). Thereafter, the CO2 refrigerant is expanded, fed to the evaporator, completely evaporated to a gaseous state in the evaporator and then circulated to the compressor.
On the other hand, when the temperature of water being supplied to the gas cooler is high, for example, about 50° C., CO2 refrigerant is heat-exchanged with high temperature water of about 50° C., and thus the CO2 refrigerant is not so cooled. Therefore, it is fed to the exit of the gas cooler while being kept under a high-temperature gas state, expanded and then circulated to the evaporator and further to the compressor.
There is a problem in that when the temperature of water being supplied is varied and the temperature at the exit of the gas cooler is varied, the density of the refrigerant is varied.
When the refrigerant density is greatly varied, there occurs a difference in the optimum amount of refrigerant being circulated between the case where the supply water temperature is high and the case where the supply water temperature is low because the volume of the refrigerating cycle is constant. When some difference occurs in the optimum refrigerant amount, if refrigerant is filled so as to support a driving operation which requires a large optimum refrigerant amount, there would occur various problems such as surplus refrigerant occurring in a driving operation requiring a small optimum refrigerant amount and, thus, a receiver tank or the like for temporarily stocking the surplus refrigerant is needed, that the control for achieving the optimum refrigerant amount is complicated, etc.
Furthermore, in this type of heat pump hot water supply apparatus, parts constituting the refrigerating cycle are disposed as a heat pump unit outdoors in many cases, and it is frequently required to carry out a defrosting operation on the evaporator. In this case, the defrosting operation has generally used a so-called “hot gas” defrosting operation in which the refrigerant discharged from the compressor is directly supplied to the evaporator while bypassing the gas cooler and the pressure-reducing device to heat the evaporator with the heat of the refrigerant, thereby defrosting the evaporator.
However, for example, when the hot gas defrosting operation is carried out under a condition such that the outside air temperature is extremely low and extremely fine powdery snow is falling, there may be a case where the amount of powder snow attached to the evaporator is excessively large and, thus, the evaporator cannot be sufficiently defrosted.
Furthermore, it is common that the gap between the bottom plate (drain pan) of the heat pump unit and the lower portion of the evaporator is extremely narrow. There may be a case where drain remaining in the gap is frozen and ice is grown up from the lower side with the frozen drain as a shell. In such a region in which some days on which the temperature drops below 0 degree centigrade are continued, it is difficult to efficiently remove ice thus grown up.
It may be considered that an electric heater or the like is disposed to perform defrosting, however, this increases the contract demand of electric power and thus increases the operating cost.
Therefore, an object of the present invention is to solve the problem of the prior art described above and provide a heat pump apparatus that can suppress a variation of an optimum refrigerant amount to be circulated when the temperature of supply water is increased with a simple construction and also efficiently defrost an evaporator.